Chapter 3. Optionally configure the BIOS

Some BIOSs provide support for serial
consoles. If your computer's BIOS is one of
these you should investigate the extent of the support provided.
Depending upon the extent of serial console support you may not need
to explicitly configure the boot loader to use the serial
port.

The contributors to this HOWTO have
encountered the following styles of BIOS support
for serial consoles.

Redirection of textual VGA output to the serial port

The BIOS takes the interrupt 0x10
"video" requests used to write to the screen and
sends the characters that would have appeared on the screen to
the serial port. Characters recieved from the serial port are
used to supply characters to BIOS interrupt
0x16 "read key" requests.

Any 16-bit application which uses the
BIOS functions for outputing text to the
screen and reading from the keyboard is redirected to the serial
port. This includes the BIOS itself, the boot
loader, and 16-bit operating systems (such as
MS-DOS).

When a 32-bit operating system (such as Linux, BSD or Windows NT/2000/XP) loads the 16-bit
BIOS is no longer accessible and the
BIOS can no longer be used for input and
output. The 32-bit operating system loads its own device drivers
for this purpose. These device drivers then need to provide the
redirection of console I/O to the serial
port.

If your BIOS uses this technique then
you should:

Configure the BIOS to redirect
keyboard input and video output to the serial port.

Do not configure the boot loader, as the
BIOS will redirect this 16-bit application's
input and output to the serial port.

Configure Linux
to use the serial port as a console, as Linux is a 32-bit operating
system.

BIOS configuration and power on self-test
uses the serial port

These BIOSs use the serial port for
configuration and the power-on self-test, but do not redirect the
interrupt 0x10 "video" requests interrupt 0x16
"read key" requests to the serial port.

Some BIOSs which usually redirect all
keyboard and video output to the serial port can be configured in
only to redirect BIOS input and output. Look
for a BIOS configuration option similar to
Cease redirection after boot.

If your BIOS uses this technique or you
choose to set Cease redirection after
boot then you should:

Configure the BIOS to send its output
to the serial port.

Configure the boot loader to use the serial port.

Configure Linux
to use the serila port as the console, as Linux is a 32-bit operating
system.

Redirection of graphical VGA output to
the serial port

Some graphical 32-bit operating systems do not provide
their own facilities to send console output to the serial port.
Some BIOSs attempt to overcome this shortcoming, using a
propietary serial protocol to send graphical output to a remote
serial client.

As these machines cannot be connected to from a standard
terminal emulator this facility is best left unconfigured when
using the Linux operating
system.

Configure the BIOS not to send output
to the serial port.

Configure the boot loader to use the serial port.

Configure Linux
to use the serial port as the console.

No serial port facilities

The BIOS cannot be accessed from the
serial port, so power-on self-test messages cannot be
seen.

Note that BIOS may still be able to be
configured remotely using the /dev/nvram device. This takes some
care.

Configure the boot loader to use the serial port.

Configure Linux
to use the serial port as the console.

If you need to configure the boot loader to use the serial
port then continue to Chapter 4.
Otherwise go directly to Chapter 5 to
configure the kernel; this is done by configuring the boot loader to
pass boot parameters to the Linux kernel.